Many liquid dispensing systems and apparatus utilize various forms of recirculation to ensure that there is constant liquid flow and pressure in the system whether a dispensing operation is on or off. For example, in hot melt adhesive dispensing units a manifold having a series of liquid flow passages is typically connected for communication between a pump and reservoir unit and a series of dispensing nozzles or guns. To provide for pressure relief in the event of an undesirable elevated liquid pressure in the system, a pressure relief valve is often incorporated into the manifold. This pressure relief valve has taken the form of a conventional valve member that is biased to a normally-closed position using a coil spring. In the event of a predetermined, elevated pressure in the system, the valve member is lifted from the seat and pressurized liquid, such as hot melt adhesive, can flow through the valve, including through the coil spring contained therein, and be directed back into the liquid reservoir.
A problem which has arisen with typical pressure relief valves is that various contaminants, such as charred adhesive, can become lodged within the pressure relief valve and can cause it to malfunction. Also, such pressure relief valves typically are not activated very often and, therefore, problems may exist in their operation due to the presence of stagnated liquid such as adhesive which may harden-up or cure. To solve these and other problems in this area, it would be desirable to provide a pressure relief valve which is efficiently and automatically cleaned or flushed during operation of a dispensing apparatus.